Were there ten or nine plagues?

No. There was a warning for each of the first two in the sets of 3. In other Words, the 3rd, 6th, and 9th plagues did not have a prior warning. This was a punishment for Paraoh's not having learned the lesson of the plagues for which he was warned.

Moses told Pharoah to let his (God's) people go, and since he didn't, the plagues came, ending with the death of the Pharoah's only son, as well as the death of all other Gentile first borns in Egypt.

Another answer:

The plagues WERE the warning!